Optical fiber laser amplifiers are a known technology for producing a coherent output beam of intermediate power. A variety of factors, including Stimulated Brillouin Scattering, four wave mixing, and optical damage, limit the output power of a single fiber amplifier to the range of several hundred watts. Although a laser of this power output may be useful in a variety of applications, other applications require higher output power than that available from a single fiber amplifier.
Higher powered laser systems have been constructed by assembling an array of fiber amplifiers driven by a master oscillator. The output beams from each of the fiber amplifiers are combined to produce a nominally single output beam. In general, in order for the combined beam to have good beam quality, the individual beams must be substantially parallel and collinear.
A variety of approaches have been used to combine the beams from multiple fiber amplifiers. Most of these employ lenses and prisms. One known approach employs a lenslet array, a precision-manufactured array of small lenses, typically on a single substrate, to combine the several beams into a single powerful beam.
The use of a lenslet array to combine the beams from several fiber amplifiers has several disadvantages. Systems using a lenslet array are difficult to align. Also, the lenslet array substantially attenuates each of the individual beams, because the individual beamlets unavoidably overfill the lenslet aperture and the interstitial space between the lenslets does not act as a lens. This insertion loss is sometimes referred to as a “fill-factor” power loss, and robs some of the output power of the amplifier array. The power lost in the lenslet array results in heating. It may therefore be necessary to cool the lenslet array. In addition, the lenslet array imprints the far-field pattern of the combined output beam with the Fourier transform of the lenslet array. This results in a loss of beam quality.
A diffractive optical element (DOE) may also be used to combine the beamlets from several fiber amplifiers into one output beam. A DOE is a special type of grating having a grating surface shape (i.e., grooves) constructed according to a particularly designed grating function. A DOE differs from a conventional grating in that the DOE grating is coarser (i.e., the spatial frequency of the grooves of the DOE is much lower), and the shape of the grooves in the DOE surface is important. The product of the grating groove frequency and the light wavelength of the beamlets to be combined defines a characteristic angle, or “eigenangle” measured with respect to the grating normal. Each beamlet to be combined by the DOE must impinge on the grating precisely at a multiple of the characteristic angle if the single combined beam is to have good beam quality and if no exiting satellite beamlets at other than normal incidence to the DOE are to be created. Moreover, the optical phase of each beamlet, measured relative to a central beamlet impinging normal to the DOE surface must either be zero or pi radians according to a certain recipe, in order that no exiting satellite beamlets are to be produced and that the central emerging beam have good beam quality. Deviation from this condition is termed “piston error”.
It is preferable that all of the energy exiting the DOE beam combiner be concentrated into a single beam or lobe. Two factors controlling whether this occurs are: (1) phase or “piston” error, defined above, and (2) “tilt” error, whereby one or more beamlets are incident on the DOE at angles which deviate from the aforementioned eigenangles, the values of which are determined by the grating function and the wavelength of the light being combined.
If either piston error or tilt error are present, unwanted secondary beamlets or lobes appear at the exit of the DOE. The beamlets are oriented along eigenangles or exit “orders”, again determined by the grating function and the wavelength of the light being combined as described above. This is undesirable because the secondary beamlet generation reduces power and beam quality in the central lobe. In addition, if enough energy is present in the secondary beamlets, the stray beamlets may heat or damage objects or equipment. For laser systems of good mechanical stability, tilt error can generally be corrected adequately by careful alignment of the beamlets with respect to the DOE at the time of assembly or at the beginning of operation. However, the sources of piston error, including thermal processes, vary significantly and rapidly during operation of the laser system.
Thus, the need exists for a laser system having a plurality of fiber laser amplifiers, each producing an intermediate beamlet, and a diffractive optical element that combines the intermediate beamlets to form an output beam, which laser system substantially minimizes piston error with respect to the beamlets incident on the diffractive optical element.